The present invention relates to a method of improving the corrosion resistance of carbonitrided components formed from ferrous material, which are subjected after the conventional carbonitriding process to one or more conventional oxidation treatments and, if necessary, to a conventional mechanical treatment, by coating them with a thin layer of an organic synthetic resin material.
The corrosion resistance of parts and components made of ferrous material which were carbonitrided and quenched from the carbonitriding temperature in water or oil is considerably improved over the untreated state. It is of no consequence whether the carbonitriding treatment took place in a salt bath, in gas or in plasma. Carbonitriding of ferrous objects is well understood in the art and the term is used herein in its recognized meaning.
A further increase in the corrosion resistance can be achieved if an oxidation treatment takes place following the carbonitriding. This can take place, for example, by means of a water vapor treatment in a temperature range of 500.degree. to 580.degree. C. Moreover, the oxidation following the carbonitriding can be carried out in an oxidizing salt bath, as described for example in DE patent 29 34 113. Such oxidation processes are well known in the art.
If the carbonitriding process is carried out in a salt bath, the oxidation process should follow immediately, that is, the ferrous components are to be switched in a suspended state without intermediate cooling directly from the carbonitriding bath into the oxidizing bath. If, on the other hand, the ferrous components are carbonitrided in gas or plasma, they must generally be cooled at first to room temperature and the oxidation is subsequently brought about by suspending the ferrous components in the salt bath. A considerable increase in the corrosion resistance of the ferrous parts also results in this method of procedure; however, it is less than in the case of salt bath carbonitriding with direct oxidation in the salt bath without intermediate cooling.
A further increase of the corrosion resistance of the ferrous products is possible if the oxidation treatment is followed by a mechanical surface treatment (e.g. polishing, lapping, slide grinding) and another oxidation. The corrosion resistance values achieved with this method of operation (e.g. in a salt spray test) are comparable to or better than those of qualitatively first-class galvanic coatings.
EP patent 0,077,627 teaches a method of providing carbonitrided components formed of ferrous material with an oxide layer and of then quenching them. The components can be subsequently provided with a thin coating of wax. However, this wax film does not entail any appreciable increase in corrosion resistance in practice.